1. Field of the Invention
This invention relates to a motor drive unit for performing film winding, shutter charging, release actuating and the like operation of a camera by motor drive, and more particularly to a winding up device for a camera suited for automatic winding up for the purpose of making a continuous series of frame exposures.
2. Description of the Prior Art
In general, a winding up device always used in a camera performs winding up operation while driving a shutter mechanism, film winding mechanism and the like with the help of the torque of an electric motor. Upon detection of completion of the winding operation, the torque of the motor is cut off. After the removal of stress caused by the winding torque, which is also detected, the shutter is released. Then, upon detection of the occurrence of the shutter being closed, the next following winding up operation is performed. In order to make a continuous series of frame exposures, therefore, it is of importance to remove the unwanted torque which remains in excess after the completion of the preceding winding operation, or otherwise the frame frequency must be decreased to assure smooth camera operation.
With a camera operating with a motor drive unit removably attached thereto, after a film winding and shutter cocking operation has been completed, an excessive load resulting from the large inertia of the motor and the movable parts related to the motor and also from the slow time response of signal detecting means such as that for detecting the point in time at which the winding operation is completed, remains applied on the various portions of the camera. This excessive load, when concentrated at the latching means of the shutter mechanism, will lead to a loss in accuracy of exposure control because of the lack of smooth sequence of events and in some cases to the impossibility of actuating a shutter release.
Attempts have been made to remove such unwanted torque exerted on the winding shaft of the camera either by providing a slippage clutch as arranged in the path of transmission of a driving torque from the motor to the winding shaft, or by detecting an accidental increase in the current flowing through the coil of the motor which will be encountered when an excess load is applied to the motor to cut off the electrical power supply to the motor. These conventional methods are characterized in that an excess load is permitted to enter the camera. Because of the slow response of the detecting means, it is very difficult to reduce the load to zero. In the case of the former method, the provision of the slippage clutch in the torque transmission will lead to an increase in the complexity of the winding up device which does not contribute to the minimization of the size of the device and also to the application of an excess electrical load on the motor at the initiation of operation of the slippage clutch. In the case of the latter method, it is required to detect a slight deviation of the intensity of current flowing through the coil of the motor from a specified value at which the motor operates with no load. However, the adjustment of the device to this specific value is difficult to perform. Further, during the winding operation, when an excess load resulting from other than the completion of the winding operation is applied, operation of the winding up device is stopped. Therefore, the number of different operating modes available in the conventional motor driven camera is limited to a small value.
Another disadvantage of the conventional automatic winding up device is that in order to remove the unwanted driving torque, the completion of the winding operation is followed by the initiation of a shutter release after a time interval large enough to prevent the frame frequency from being increased.
An object of the present invention is to provide a winding up device for a camera which has overcome the above-mentioned conventional drawbacks. Instead of detecting the excess load which remains after the completion of winding operation, use is made of means for detecting the point in time at which the winding operation is completed in combination with an electromagnetically activated clutch as arranged in the driving torque transmission so that a large residual driving torque is prevented from entering the camera.
Another object of the present invention is to achieve minimization of the residual driving torque which is effected during the time when the light value for automatic exposure control is stored. According to the invention, therefore, there is no need to provide any time interval particularly designated for removing the residual driving torque.